The present invention relates to an apparatus for pressure molding articles made of plastics, such as caps for closing a container and the like.
U.S. Pat. No. 5,807,592 by the same Assignee discloses apparatuses of the indicated type for the pressure-molding of caps for closing a container, such as screw caps. Such apparatuses comprise a carousel that rotates about a vertical axis and on which a plurality of pressure-molding units are mounted concentrically around the rotation axis of the carousel and at an identical mutual angular distance. Each one of said units comprises an upper punch, which cooperates with a lower mold that is aligned with said punch and has a molding cavity.
By virtue of the rotation of the carousel, the molding units trace a circular path, which comprises a first sector, in which the necessary doses of plastic material to be molded are deposited in the cavities of the molds, a second sector, in which the article is molded, a third sector, in which the molded article is cooled, and a fourth sector, in which the molded article is extracted and conveyed away.
In these known apparatuses, the plastic material to be molded is removed from an extruder by means of a rotating head provided with a plurality of removal elements, which trace a circular path that has a point of tangency with the extrusion nozzle and with the circular path traced by the molding units. The rotating head and the carousel are mutually in step, in order to allow the removal elements to remove in succession doses of plastic material from the extruder and deposit them in the cavities of the molds.
The conventional apparatuses suffer the drawback that in the time that elapses between the moment when the doses are deposited in the cavities and the moment when said doses are compressed, the portion of the dose that is introduced in the cavity, by making contact with the colder surface of the cavity, undergoes cooling and therefore a variation in the degree of plasticity of the plastic material that is located at said portion, which causes, during molding, aesthetic defects that can be observed on the outer surface of the molded article. These defects, which become apparent mostly in the form of regions whose surface differs in terms of opacity from the surrounding regions and which sometimes have a certain surface porosity, constitute an unacceptable qualitative depreciation of the molded article.
Another drawback is the fact that the doses usually are not deposited at the center of the cavities of the molds, and therefore during the compression step the distribution of the plastic material does not expand uniformly, since it does not start from the center, causing molding problems due to the asymmetry of the filling.
Other drawbacks due to the deposition of the doses in the molding cavities can be observed in apparatuses (see U.S. Pat. No. 5,885,408) for forming plastic caps, on the bottom of which a label for decorative purposes or bearing information is to be incorporated externally. In these apparatuses, the labels are deposited inside the molding cavity before the doses, so that said doses, when they make contact with the label, due to their high temperature, cause deformations of the labels, particularly creases, which are unlikely to be smoothed out during the compression step.